This invention relates to a method and apparatus for purifying reverse osmosis treated water and more particularly to a method for removing hydrogen sulfide (H.sub.2 S) and subsequently adjusting pH and hardness. Reverse osmosis (RO) water treatment plants are often fed with water which contains hydrogen sulfide (H.sub.2 S). The H.sub.2 S passes through the membranes and into the product water (or permeate), where it must be removed prior to distribution as potable water. The traditional method of accomplishing this degassification is via forced draft degassifiers, sometimes followed by a gas scrubber to clean the gasses prior to release to the atmosphere. This degassification/scrubbing process is not 100% efficient and hydrogen sulfide emissions and resulting odor problems inevitably occur. However, the degassification process also creates a new problem; it saturates the water with oxygen, which leads to subsequent downstream corrosion problems in the copper pipes in the homes of consumers. More specifically, scattered incidences of copper pipes developing holes frequently occur.
Examination of sections of the corroded and failed copper pipes indicates a pitting type of corrosion, as opposed to a grooving type. The pits were distributed over the entire inside walls of the failed pipes. The pits were of varying sizes and depths, with the deepest ones eventually leading to holes and leaks. This type of corrosion is caused by oxygen in the water resulting from the forced draft degassification.
Common methods for removing oxygen from water include the addition of sodium sulfite (Na.sub.2 SO.sub.3) or hydrazine (N.sub.2 H.sub.4) to the processed water. Vacuum degassification is another possibility. All these oxygen removal techniques result in either unacceptable water quality or excessively high costs.
Another potential solution to the corrosion problem is to avoid the oxygen addition altogether and remove the H.sub.2 S by other means than degassification. Oxidation of the H.sub.2 S to other forms is one such possibility. Ozone is one potential oxidizing chemical, but it is too costly.
Another chemical that may be used as an oxidizing agent is chlorine. However, the potential problem with chlorine oxidation of H.sub.2 S is that there are two possible chemical paths . . . one of which could cause problems in a potable water system. The two potential paths can be described by the following two chemical equations: EQU H.sub.2 S+Cl.sub.2 =2HCl+S (1) EQU H.sub.2 S+4Cl.sub.2 +4H.sub.2 O=H.sub.2 SO.sub.4 +8HCl (2)
Equation (1) shows the formation of elemental sulfur, which would create turbidity and foul downstream equipment. If this occurred, removal and disposal of the sulfur would obviously be required, and this would result in very high capital and operating costs. Equation (2) shows all the H.sub.2 S oxidized to sulfate (SO.sub.4), which remains in solution as a harmless anion.
Historically, the problem has been how to avoid the reaction in Equation (1). This has proven to be very difficult, and chlorine oxidation of H.sub.2 S has therefore been largely limited to situations where the elemental sulfur poses no serious problem, as in irrigation systems (where the sulfur is discharged along with the water itself) or in the raw water feed to water treatment plants (where filtration or other methods are then used to remove the elemental sulfur). For chlorine oxidation to be a practical and economic process for eliminating the H.sub.2 S in reverse osmosis permeate, a way would have to be found to avoid the reaction in Equation (1) and to force Equation (2) to prevail.
Oxidation of H.sub.2 S in the incoming feed water has been tried many times in the utility industry. In all cases, elemental sulfur was formed and either it was removed (by filtration, etc., at considerable expense) or the process was subsequently abandoned. Chlorine is not used to remove H.sub.2 S from the feed water to reverse osmosis plants because of the need for extremely low turbidity to avoid membrane fouling.